The present invention is directed to a process effective in removing sulfur compounds from gas or liquid feed streams, in particular, hydrocarbon streams such as natural gas and refinery process streams, nitrogen gas streams and other feed streams. More particularly, the present invention is directed to a process which utilizes a regenerable absorbent for removing sulfur compounds which include sulfur in the negative two (xe2x88x922) oxidation state from feed streams containing these sulfur impurities.
Hydrocarbon streams, such as natural gas and refinery process streams, contain a wide range of impurities which are removed for any of a variety of reasons, such as for health and/or environmental safety, and/or for process operability or reliability. Among the impurities present in these streams are sulfur compounds, in particular, reduced sulfur compounds, such as hydrogen sulfide (H2S), mercaptans (designated generally as Rxe2x80x94SH compounds), dialkyl sulfides (designated generally as R1xe2x80x94Sxe2x80x94R2 compounds), carbonyl sulfide (COS), carbon disulfide (CS2) and thiophenes. All of these compounds include sulfur in an oxidation state of (xe2x88x922). Other impurities typically contained in these streams and removed for one or more of the above mentioned reasons include H2O, N2, and CO2.
Several processes are known for removing sulfur containing impurities from hydrocarbon streams. These processes are commonly referred to as processes for sweetening sour hydrocarbon streams.
U.S. Pat. No. 3,449,239 discloses a process in which a sour hydrocarbon stream is contacted with a sweetening reagent, air and a diazine, such as piperazine. Suitable sweetening reagents are disclosed as including aqueous caustic solution and methanol, coupled with a metal phthalocyanine catalyst (for example, cobalt phthalocyanine or cobalt phthalocyanine disulfonate). According to the disclosure, the sweetening reaction comprises converting mercaptan to dialkyl disulfide through an oxidation reaction, and then removing disulfide from the stream. It is to be noted that dialkyl sulfides cannot be converted to dialkyl disulfides and thus may not be removed efficiently by this process.
U.S. Pat. No. 4,336,233 discloses processes for washing natural gases, coke-oven gases, gases from the gasification of coal and synthesis gases with aqueous solutions containing a specific amount of piperazine, or with a specific amount of piperazine in a physical or chemical solvent. The use of a specific concentration of piperazine is reported for the purpose of removing sulfur impurities such as H2S, CO2 and COS. Among the physical solvents disclosed are mixtures of dialkyl ethers of polyethylene glycols (e.g., SELEXOL solvent available from Union Carbide Corporation, Danbury, Conn.). The preferred chemical solvent is monoalkanolamine. According to the description in the ""233 patent, COS can only be partially removed by the process. In order to achieve more complete removal, COS must first be converted by hydrogenation into more readily removable compounds (CO2 and H2S). These sulfur compounds are then removed by solvent absorption.
U.S. Pat. Nos. 4,553,984, 4,537,753, and 4,997,630 also disclose processes for removing CO2 and H2S from gases. Each patent discloses removing CO2 and H2S by treating the gas with an aqueous absorption liquid containing methyldiethylanolamine. The absorbed H2S and CO2 is then removed from the absorbent in one or more flashing stages and/or a steam stripping tower.
As mentioned above, liquid streams containing sulfur impurities are also subjected to treatment in an effort to reduce or eliminate sulfur impurities. One such process is disclosed in U.S. Pat. No. 5,582,714. The ""714 patent discloses a process for reducing the sulfur content in petroleum fractions such as FCC (fluid catalytically cracked) gasoline by employing, for example, polyalkylene glycol and/or polyalkylene glycol ethers having a molecular weight of less than 400. The process requires the steps of treating the hydrocarbon stream with the solvent to produce a sulfur depleted hydrocarbon phase and a sulfur rich solvent phase, stripping the sulfur containing impurities from the solvent, separating the stripped sulfur containing stream into a sulfur rich component and an aqueous phase, washing the sulfur depleted hydrocarbon phase with the aqueous phase to remove any solvent from the sulfur depleted hydrocarbon phase, and then returning the washed solvent to the treating step.
Like the ""714 patent, U.S. Pat. No. 5,689,033 is directed to processes for reducing impurities in liquid hydrocarbon feedstocks. More specifically, the process disclosed in the ""033 patent involves removing sulfur compounds, oxygenates and/or olefins from C4-C6 fractions using lean solvents such as diethylene and/or triethylene glycol, certain butane glycols, and/or water or mixtures of these solvents. Thereafter, the removed compounds are stripped from the impurities-rich solvent stream.
These prior art processes reduce the content of sulfur containing compounds in hydrocarbon feed streams to some extent; however, each process exhibits significant shortcomings. Solvents such as aqueous alkanolamines or caustic, which work on the basis of a Bronsted acid/base reaction, are unable to remove dialkyl sulfides efficiently and are unable to slip CO2, which in some cases is very desirable. Some, like the processes disclosed in the ""239 patent and the ""233 patent require a chemical reaction to convert sulfur containing impurities such as mercaptan and COS to other sulfur containing compounds which are more amenable to removal by solvent extraction. Other prior art processes employ a variety of solvents to solubilize the sulfur containing compounds, followed by elaborate chemical and water washing and stripping processes. These latter processes are not particularly effective in removing sulfur compounds, and also suffer from the drawback of removing valuable hydrocarbon fractions from the stream. Moreover, in some instances, these processes can be unstable, causing, for example, foaming to occur in the equipment used to treat the feed stream.
It is, therefore, an object of the invention to provide a process which is capable of removing sulfur containing compounds from gas and liquid feed streams containing these impurities without the need for a chemical reaction to convert the compounds to a more easily removable form.
It is a further object of the invention, in the case of hydrocarbon feed streams, to provide such a process which does not require the use of solvents that solubilize valuable hydrocarbons together with the sulfur compounds.
It is yet another object of the invention to provide such a process which utilizes an absorbent that is readily regenerable simply by heating and/or stripping.
It is still another object of the invention to provide a process which is highly selective for the removal of sulfur compounds having sulfur in the (xe2x88x922) oxidation state while not significantly absorbing CO2 that may also be present in the feed stream.
The invention meets these objects by providing a process which utilizes a regenerable absorbent that is selective essentially exclusively for sulfur compounds including sulfur in the (xe2x88x922) oxidation state. According to the process taught by the invention, a feed stream containing at least one sulfur compound including sulfur in a (xe2x88x922) oxidation state is contacted with a metal cation-containing organic composition to form with the sulfur compound a plurality of sulfur-metal cation coordination complexes in which the oxidation state of the sulfur and the metal cation remains essentially unchanged. The complexes are separated from the feed stream, and the absorbent is then regenerated by disassociating the sulfur compound from at least some of the plurality of coordination complexes. At least a portion of the regenerated absorbent is then recovered for additional use in removing sulfur compounds which include sulfur in an oxidation state of (xe2x88x922) from a feed stream containing such compounds.
As presently understood, and without intending to limit the scope of the present invention, it is believed that the absorbent utilized in the process functions essentially as a Lewis acid (electron acceptor) to form with the sulfur compound, acting as a Lewis base (electron donor), the sulfur-metal cation coordination complexes in which neither the metal cation nor the sulfur exhibits any permanent change in formal oxidation state. By essentially maintaining the oxidation state of the metal cation and the sulfur unchanged through a complexation mechanism, the sulfur compound can be separated from the absorbent, and the absorbent thereby regenerated, by simple thermal treating and/or stripping.
Preferably, the sulfur compound is contacted with an absorbent comprising a metal cation-containing phthalocyanine or porphyrin composition capable of forming sulfur-metal cation coordination complexes with sulfur compounds containing sulfur in a (xe2x88x922) oxidation state. Most preferably, the absorbent comprises a metal cation-containing phthalocyanine composition wherein the metal cation is either iron or copper.
In a preferred embodiment of the invention, the absorbent is dissolved in water or dissolved or suspended in any one of a number of solvents commonly employed in a variety of known processes used to treat feed streams, particularly hydrocarbon feed streams, contaminated with acid gases such as CO2 and H2S and containing sulfur compound having sulfur in the (xe2x88x922) oxidation state. Such known solvents include aqueous amine solutions which usually include one or more alkanolamines, such as triethanolamine (TEA), methyldiethanolamine (MDEA), diethanolamine (DEA), monoethanolamine (MEA), diisopropanolamine (DIPA), hydroxyaminoethyl ether (DGA), and piperazine. Known organic solvents include those comprising a mixture of dialkyl ethers of polyalkylene glycols, such as SELEXOL solvent. The absorbents taught by the invention may also be used with other well known aqueous and organic solvents typically used in the art to treat contaminated liquid and gas feed streams.